Team creates building block for quantum networks

Scientists say they've built a proof-of-concept device that could become the memory or processing element for an on-chip optical quantum network.

It combines a single nitrogen-vacancy centre in diamond with an optical resonator and an optical waveguide. A nitrogen-vacancy centre is a defect in the lattice structure of diamond where one of the carbon atoms is replaced by a nitrogen atom and the nearest neighbour carbon atom is missing, and has the property of photoluminescence.

The photons it emits are special in that they are correlated, or entangled, with the nitrogen-vacancy centre that they came from.

In the device, the photons are produced from a nitrogen-vacancy centre within a diamond microring resonator. The nitrogen-vacancy centre is located inside the diamond resonator as it is more likely to emit photons than when it is located in the waveguide or just in plain diamond. Moreover, the photons emitted in the resonator are easier to couple into an on-chip waveguide.

The cotton bud-shaped waveguide sends the photons out into a desired direction through gratings at either end.

"One of the holy grails in quantum photonics is to develop networks where optical quantum emitters are interconnected via photons," says lead author of the study Andrei Faraon of Caltech.

"In this work we take the first step and demonstrate that photons - the information carriers - from a single nitrogen-vacancy centre can be coupled to an optical resonator and then further coupled to a photonic waveguide. We hope that multiple devices of this kind will be interconnected in a photonic network on a chip."

The entire device was etched in a diamond membrane that was around 300 nanometres thick.The team tested it by cooling it to below 10K and shining a green laser onto the nitrogen vacancy to evoke photoluminescence.

"The whole idea of these devices is that they are able to be produced en masse. So far the procedure for mass fabrication is still at the proof-of-concept level, so there is still plenty of work to be done to make it reliable," says Faraon.